college physics
posted by anne .
An object acted on by three forces moves with constant velocity. One force acting on the object is in the positive x direction and has a magnitude of 6.5 N; a second force has a magnitude of 3.9 N and points in the negative y direction. Find the direction and magnitude of the third force acting on the object.

Newton's second law states that for every action there is an equal and opposite reaction. Draw a force diagram. It is moving at a constant velocity. Your force diagram for an object moving at a constant velocity is the same as your force diagram for an object at rest, so you can solve the same way. Negative y fOrce of gravity always exists, so the force you are looking for is keeping it up.
I'm not goig to just give you the answer. (ps, I'm a freshman in my 5th week of highschool and I'm in physics, but since we just learned what you're learning, I thought I'd help you. 
that did not help at all

F1+F2+F3 = M*a = M*0 = 0.
6.5  3.9i + F3 = 0.
F3 = 6.5 + 3.9i = 7.58 N.[31o] N. of W.
= 149o CCW from +xaxis.